Up to the development of low pressure methanol synthesis processes dimethylether has been obtained in a quantity of 2 -5 weight-% as a side-product in high-pressure methanol production units and has been isolated by distillation from methanol-fractions, which contained additional low boiling side-products.
After introduction of low pressure methanol processes, which yield only negligible quantities of dimethylether, special synthetic processes have been developed, based on the catalytic dehydration of methanol. Numerous processes have been disclosed in the patent literature. For example, according to DE-PS 680 328, aliphatic ethers are obtained by heating alcohols in the presence of zinc chloride.
Other suitable catalysts for the production of ethers from alcohols are according to GB-PS 332 756, GB-PS 350 010, GB-PS 403 402, US-PS 1 873 537 and FR-PS 701 335, ferrous and ferric chloride, copper sulfate, stannic and stannous chloride, manganese chloride, aluminium chloride and -sulfate, chromic sulfate, alum, thorium compounds, aluminium oxide, titanium oxide, barium oxide, silica or aluminium phosphate.
In "Industrial and Engineering Chemistry", Vol. 41, No. 12, page 2928 (1949) use of bauxite with a SiO.sub.2 portion of 4,40 -13,99 weight-% is described. In US-PS 3 036 134 an aluminium silicate-catalyst is disclosed for the production of dimethylether from methanol, with a ratio of Al.sub.2 O.sub.3 :SiO.sub.2 of 1 part: 1,35-0,3 parts. The synthesis gas (CO+H.sub.2) has also been described (DE-PS 23 62 944, DE-PS 27 57 788 and DE-PS 32 20 547).
The technically most important catalysts have turned out to be according to DE-PS 28 18 831, DE-OS 32 01 155, EP-A 0 099 676 and EP-A 0 124 078 in particular aluminium oxide and aluminium silicate catalysts with and without doping. In DE-PS 28 18 831 a catalyst for the production of dimethylether is disclosed, which can contain any aluminium oxide as a base material, as far as it possesses a sufficiently large surface and additives of 1 to 30 weight-% of rare earthes.
In EP-A 0 099 676 a catalyst is disclosed, which contains 1-20 weight-% of SiO.sub.2, preferably 1-10 weight-% of SiO.sub.2 and more preferably 6 weight-% of SiO.sub.2.
Crude dimethylether thus obtained contains reaction water, unreacted methanol as well as small quantities of contamination, like methyl formate, hydrocarbons, aminesand sulfides, carboxylic acids and esters, amides, acetales and others.
In EP-A 0 270 852 a process is described, according to which dimethylether is produced from methanol in the presence of a catalyst, which contains very small quantities of SiO.sub.2.
In synthesis units for the production of DME, crude dimethylether is worked up in two distillation columns connected in series. In the first column, which operates under pressure, pure dimethylether is obtained. In the second one unreacted methanol is distilled off. Thus in EP-A 0 124 078 a process is described, according to which dimethylether is drawn off as a sidestream in a first column, which is operated under pressure, whereas in a second column, which is operated under lower pressure, contaminations with boiling points between dimethylether and methanol, are drawn off overhead. Methanol is obtained in the same column as a sidestream. Catalysts are Al.sub.2 O.sub.3, SiO.sub.2, aluminum silicates and preferably .delta.-Al.sub.2 O.sub.3.
Although by this process dimethylether of higher purity compared to the state of the art is obtained, it has, from an economical point of view, a considerable disadvantage, because not only the first column, but also the second one have to be equipped with a high number of trays. This leads to high investment costs and in particular to high costs of operation. Furthermore, there exists a considerable risk, that the contaminations, boiling between dimethylether and methanol, are not completely transferred to the second column, but accumulate in the first column and are drawn off with dimethylether As a consequence a dimethylether quality is obtained, which is not free of odor.
In EP-A 0 270 852, already mentioned above, a purification process for the production of highly pure, odorless dimethylether by distillation is disclosed, characterized by withdrawal of a fraction, which contains contaminations boiling between dimethylether and methanol, in the same column from which pure dimethylether is distilled off. This fraction has to be withdrawn at defined trays as a side-stream.
Since dimethylether gains increasing importance as a propellant for sprays, very high demands are made with regard to purity. Thus, no irritating substances in dimethylether are permitted in applications like cosmetic, human and household sprays. Furthermore dimethylether has to be free of odor for these applications.
Object of the instant invention therefore was the production of a highly pure odorless dimethylether, which is also obtained by a more economical process compared to the state of the art and to convert methanol nearly quantitatively into a highly pure product which is suitable for the above named applications.